A variety of techniques and instruments have been developed for use in the removal or repair of tissue in arteries and similar body passageways. A frequent objective of such techniques and instruments is the removal of atherosclerotic plaques in a patient's arteries. Atherosclerosis is characterized by the buildup of fatty deposits (atheromas) in the intimal layer (under the endothelium) of a patient's blood vessels. Very often over time, what initially is deposited as relatively soft cholesterol-rich atheromatous material hardens into a calcified atherosclerotic plaque. Such atheromas restrict the flow of blood, and therefore are often referred to as stenotic lesions or stenoses, the blocking material being referred to as stenotic material. If left untreated, such stenoses can cause angina, hypertension, myocardial infarction, strokes and the like.
Several kinds of atherectomy devices have been developed for attempting to remove some or all of such stenotic material. In one type of device, such as that shown in U.S. Pat. No. 5,092,873 (Simpson), a cylindrical housing, carried at the distal end of a catheter, has a portion of its side-wall cutout to form a hollow housing into which the atherosclerotic plaque can protrude when the device is positioned next to the plaque. An atherectomy blade, disposed within the housing, is then advanced the length of the housing to lance the portion of the atherosclerotic plaque that extends into the housing cavity. While such devices provide for directional control in selection of tissue to be excised, the length of the portion excised at each pass of the atherectomy blade is necessarily limited to the length of the cavity in the device-in turn, the length and relative rigidity of the housing limits the maneuverability and therefore the utility of the device in narrow and tortuous passageways such as coronary arteries.
Another approach which solves some of these problems involves the use of a rotating burr covered with an abrasive cutting material such as diamond grit (diamond particles or dust) carried at the distal end of a flexible drive shaft, similar to a dental abrading/polishing tool. Examples of such devices are illustrated in U.S. Pat. No. 4,990,134, (issued to Auth), as well as "Premier Two Striper.RTM. Gingival Curettage" (Abrasive Technology, Inc. 1982); "Premier Two Striper.RTM. Crown & Bridge Techniques" (Abrasive Technology, Inc. 1981); H. Gilmore, et. al, Operative Dentistry (C.V. Mosby Company 1982, 4th ed.), pp. 64-65, 69, 348-350; R. Tupac, et al., "A Comparison of Cord Gingival Displacement With the Gingitage Technique," Journal of Prosthetic Dentistry, (Nov. 1981, pp. 509-515); and Premier Presents Two Striper.RTM. Dental Diamond Instruments (Abrasive Technology, Inc. 1989). The burr in such devices is rotated at speeds in the range of 20,000 to 200,000 rpm or more, which, depending on the diameter of the burr, can provide surface speeds of the abrasive particles on the burr above or below 40 ft/sec. Auth claims that at surface speeds below 40 ft/sec the abrasive burr will remove hardened atherosclerotic material but will not damage normal elastic soft tissue of the vessel wall. Auth also admits that at surface speeds above 40 ft/sec the abrasive burr will remove both hardened and soft tissue. See, e.g., U.S. Pat. No. 4,990,134 at col. 3, lines 20-23. Unfortunately not all atherosclerotic plaques are hardened, calcified atherosclerotic plaques. Moreover, the mechanical properties of the soft plaques are very often quite close to the mechanical properties of the soft wall of the vessel. Thus, one cannot safely rely entirely on the differential cutting properties of such abrasive burrs to remove atherosclerotic material from an arterial wall, particularly where one is attempting to entirely remove all or almost all of the atherosclerotic material. See, e.g., Atherectomy, A Physicians Guide, (Strategic Business Development, Inc., 1990), pp. 89, 94-96. Furthermore, the Auth burr effectively blocks blood flow through the artery during the passage of the burr through the stenosis, thus limiting the amount of time of each pass across the stenosis to less than one minute (and perhaps as little as 10 seconds). See id. at pp. 95- 96. Because the size of the particles removed by the Auth burr is very small (typically 5 microns or less), and because of the time limitations described above, in clinical practice, in order to remove a sufficient amount of tissue during each pass of the burr across the stenosis, the Auth burr is virtually always rotated at speeds of at least about 155,000 rpm. At such speeds a diamond dust covered burr with a diameter of 1.5 mm achieves a surface speed of 40 ft/sec, the very speed at which the differential cutting effect becomes limited, at best (i.e., the burr removes both hard and soft tissue).
The ability of diamond dust covered burrs to remove human soft tissue at high surface speeds (e.g., small diameter burrs rotated at about 200,000 rpm) has been known for some time and has been utilized in dentistry since at least the early 1980's to remove soft gum tissue (see, e.g., "Premier Two Striper.RTM. Gingival Curettage" (Abrasive Technology, Inc. 1982); "Premier Two Striper.RTM. Crown & Bridge Techniques" (Abrasive Technology, Inc. 1981); H. Gilmore, et. al, Operative Dentistry (C.V. Mosby Company 1982, 4th ed.), pp. 348-350; R. Tupac, et al., "A Comparison of Cord Gingival Displacement With the Gingitage Technique," Journal of Prosthetic Dentistry, (Nov. 1981, pp. 509-515).
Several problems have been recognized in use of the Auth-type of burr, however. First, although under some conditions the differential cutting properties of such burrs are effective to protect healthy tissue, in many circumstances the burr nevertheless can abrade at least a portion of the healthy tissue, creating a risk of perforation. This is particularly true at higher rotational speeds. A majority of atherosclerotic lesions are asymmetrical (i.e., the atherosclerotic plaque is thicker on one side of the artery than on the other). Moreover, pressure of the burr against the atherosclerotic plaque is achieved only by the use of a burr having a diameter slightly larger than the opening through the stenotic passageway. Thus, since the stenotic material will be entirely removed on the thinner side of an eccentric lesion before it will be removed on the other, thicker side of the lesion, during removal of the remaining thicker portion of the atherosclerotic plaque the burr necessarily will be engaging healthy tissue on the side which has been cleared--indeed, lateral pressure by such healthy tissue against the burr is required to keep the burr in contact with the remaining stenotic tissue on the opposite wall of the passageway. For stenotic lesions that are entirely on one side of an artery (a relatively frequent condition), this means that the healthy tissue across from the stenotic lesion will be exposed to and in contact with the abrasive burr for substantially the entire procedure. Moreover, pressure from that healthy tissue against the burr will be, in fact, the only pressure urging the burr against the atherosclerotic plaque. Under these conditions, a certain amount of damage to the healthy tissue is almost unavoidable, even though undesirable, and there is a clear risk of perforation. Thus, in clinical practice (balancing safety and residual stenosis), physicians rarely use a burr diameter of more than 2 mm, even on patients where the original diameter of the coronary artery lumen is estimated to be 3 mm. See, e.g., Atherectomy, A Physicians Guide, (Strategic Business Development, Inc., 1990), p. 96. These risks are enhanced at high rotational speeds where the differential cutting phenomenon is significantly diminished.
As indicated above, in clinical practice the opening of the stenosis of coronary arteries using the Auth-type burr is performed very fast and thus very large numbers of small particles of stenotic material (estimated to be 1,000,000 per cubic mm of stenotic material removed--see id. at p. 92) are released into the coronary artery within a very short period of time. Although individually the particles (typically in the range of 5 microns) can easily pass through the capillaries, when such large numbers of such particles are released within a very short period of time it is very possible that there is a risk that they may at least temporarily occlude the capillaries. This may explain the heart pain which is not infrequently experienced by patients immediately after the Auth-type burr is passed across the stenosis, as well as elevated levels of enzymes indicative of myocardial ischemia (such as CPK) which have been documented in some patients after the Auth-type burr procedure. See id. at p. 95. In a prior patent application of Applicant's, Ser. No. 07/967,765, filed Oct. 28, 1992, incorporated herein by reference (the "'765 application"), Shturman described a rotational atherectomy apparatus with a special guide wire that gives directional control over the position of the atherectomy device's abrasive burr within a body passageway, thereby providing a method for controlled removal of tissues from body passageways, such as atherosclerotic plaques from arteries. The Shturman rotational atherectomy apparatus, as with the Auth-type device, includes a rotatable abrasive burr carried at a distal end of a flexible, elongated drive shaft. The drive shaft in turn typically is disposed in a catheter having a central lumen, and is connected to means for rotating the drive shaft at high speeds. The special guide wire of the Shturman rotational atherectomy apparatus comprises a flexible, generally straight proximal portion, a flexible distal tip portion, and a flexible intermediate burr-positioning segment with a predetermined curved shape. This curved intermediate burr-positioning segment controls the lateral position of the burr within the body passageway and is, in fact, responsible for directional control over the rotational atherectomy device. The guide wire is sized so that its proximal end can be inserted into the distal end of the drive shaft lumen, permitting the drive shaft and burr to be advanced longitudinally over the guide wire to locate the burr along the predetermined shape of such intermediate burr-positioning segment, thereby selectively positioning the burr laterally away from the longitudinal axes of the proximal and distal portions of the guide wire.
The lateral position of the burr of this Shturman device within the body passageway is controlled by the degree of lateral deflection and the orientation of the curve of the intermediate burr-positioning segment of the guide wire. Thus, by advancing or retracting the drive shaft and burr with respect to the guide wire, the burr can be selectively located along the burr-positioning segment of the guide wire to selectively position the burr laterally of the axes of the proximal and distal portions of the guide wire. Guide wires having intermediate burr-positioning segments with different shapes can be used to control both the lateral position and the angular orientation of the burr, providing great flexibility in positioning the burr to remove unwanted tissue (such as an atherosclerotic plaque).
The Shturman apparatus described in the '765 application provides several distinct advantages over devices such as the Auth and Simpson atherectomy devices. With respect to Auth-type devices, there are a number of advantages to the Shturman apparatus:
1. It provides directional control over the removal of stenotic tissue, reducing the risk of damage to or perforation of the normal vascular wall;
2. It provides a small diameter burr-based instrument capable of opening stenoses in large diameter peripheral arteries (such as the femoral and iliac arteries) without resorting to entry through a cut-down on the femoral artery;
3. It does not completely occlude the blood flow through an artery during an atherectomy procedure, thus not limiting the time available to the physician to open the stenosis; and
4. It provides for slower, controlled release of particles of stenotic material into the capillaries over a longer period of time.
With respect to the Simpson-type atherectomy devices, the Shturman apparatus provides at least two additional advantages:
1. It is very flexible (compared to Simpson-type devices which typically have a rigid housing), permitting its use in small, more tortuous arteries: and
2. It provides the ability to remove hard, calcified stenotic tissue.
In another Shturman application Ser. No. 07/962,634, filed Oct. 16, 1992, incorporated herein by reference (the "'634 application"), Shturman described yet another device for directional rotational atherectomy where the lateral positioning of the abrasive burr is controlled not by a burr positioning segment of a guide wire, but by a specially designed positioning wire which also has a burr-positioning segment. This second Shturman device includes an elongated catheter having at least two lumens, and the guide wire and positioning wire are located inside these separate lumens. Distally of the catheter the positioning wire is slidably attached to the guide wire. Compared to the first-mentioned Shturman apparatus (the '765 application), this second Shturman device provides two significant additional advantages over the Auth-type atherectomy device:
1. It allows one to widely open even large coronary arteries to their large original diameter without significant residual stenosis using a single directional rotational atherectomy instrument rather than multiple instruments with successively larger diameter burrs (which often still leave significant residual stenosis);
2. It permits use of intravascular ultrasound imaging to image a cross-section of the stenotic area (including the thickness and composition of the atherosclerotic plaque), and the relative position of the abrasive burr with respect to the stenotic tissue. The intravascular ultrasound imaging permits monitoring of the removal of the stenotic tissue as it is being removed, thus further enhancing the safety of the procedure.
Both of the Shturman devices and the Auth device have several drawbacks due to the fact that a separately manufactured abrasive burr must be attached to (or near) the distal end of the flexible drive shaft. First, the connection between the burr and the drive shaft is critical, in that it must be secure against failure. This requirement therefore adds to the cost of producing the device.
Second, the size of the burr necessarily limits the ability of the device to be advanced into and through small diameter and/or tightly stenotic arteries.
Third, friction of the diamond dust covered burr against the stenotic tissue results in a frictional force. The torque applied to the rotating drive shaft due to this frictional force is generally proportional to the lever arm (or moment arm) of the frictional force, and thus is always larger for a drive shaft with a burr than for the same drive shaft without a burr since the diameter of the burr mounted on the drive shaft is always larger than the diameter of the drive shaft itself. Thus, it would be desirable to find a way to reduce the lever arm of the frictional force acting on the drive shaft, thereby reducing the torque acting on the drive shaft and thus reducing the risk of drive shaft failure.